1. Field of the Invention
The present invention relates to a compatible optical pickup apparatus. More particularly, the present invention relates to an optical pickup apparatus having a simple optical system and is capable of compatibly employing a plurality of optical recording mediums having different recording densities.
2. Description of the Related Art
In an optical recording/reproducing apparatus which records and reproduces information on and from a storage medium such as an optical disc using a light focused by an objective lens, recording capacity is determined by a size of an optical spot on the storage medium. The size of the optical spot is determined by a wavelength (λ) of the light being used and a numerical aperture (NA) of the objective lens, as expressed in Equation 1 below.S∝λ/NA  Equation 1
Therefore, in order to reduce the size of the optical spot formed on the optical disc and provide greater storage density on the optical disc, a short-wavelength light source such as a celadon laser and an objective lens having an NA of at least 0.6 should be employed.
A digital versatile disc (DVD) uses a light having a wavelength of 650 nm (or 635 nm) and an objective lens having a NA of 0.6 or, 0.65 when a DVD-recordable (R) is used to record and reproduce the information. Capacity of a DVD, having a diameter of 120 mm and a track pitch of 0.74 μm, is roughly 4.7 GB per side.
Also, a Blue-ray disc (BD) has been developed as an advanced high-density optical recording medium which uses an objective lens having a large NA, such as, 0.85.
When using the BD, a blue light source having short wavelength (405 to 408 nm) may be used. A thickness of the disc may be 0.1 mm. Such a BD can have nearly 10 times as much storage capacity as the DVD. However, compatibility with conventional storage mediums or playback devices is important when developing new optical recording medium.
For example, since the DVD-R and a compact disc (CD)-R, which are conventional once-writable optical discs, have poor reflexibility, light sources which project light having wavelengths of 650 nm and 780 nm should be employed. In consideration of compatibility between the DVD-R and the CD-R, an optical pickup for the BD needs to employ three light sources having different wavelengths and a plurality of objective lenses having different NAs.
However, fabrication of such an optical pickup, which is compatible with CDs, DVDs and BDs, requires a lot of component parts, and this complicates the structure of the optical pickup.
Accordingly, there is a need for an optical pickup apparatus having a simple optical system that is compatible with different optical mediums having different recording densities.